Process for chlorine bleaching of kraft pulp using a sulfite extraction stage

ABSTRACT

Extraction of chlorine bleached kraft wood pulps with aqueous peracids at pH values of 7 or below, aqueous sulfites at pH values below 10 or with certain non-aqueous solvents provides extracted pulps having lower residual color and permits use of lesser quantities of alkali or chlorine dioxide in subsequent bleach steps to attain brightness and strength levels comparable to those attained with pulps treated by alkaline extraction at pH values of 10 or higher. At a comparable level of alkali and chlorine dioxide or hypochlorite application, extraction of chlorinated pulp with peracids provides bleached pulps with enhanced brightness.

This application is a continuation of application Ser. No. 546,263,filed Oct. 28, 1983, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the art of chemical pulping and bleaching ofwood pulp, particularly the art of removal, by chemical means, ofresidual lignin remaining in wood pulp produced by chemical pulpingprocesses, more specifically use of chlorine to remove residual ligninfrom or bleach wood pulp produced by sulfate or "kraft" pulpingprocesses.

The conventional classical multistage chlorine and chlorine dioxidebased bleaching sequence for kraft pulp is one wherein the unbleachedkraft pulp is treated with chlorine, conveniently in the presence of asmall quantity of a viscosity preserving agent, such as chlorinedioxide, then extracted with aqueous alkali at high pH values,conveniently greater than pH 10, followed by treatment with eitherhypoclorite followed by chlorine dioxide or chlorine dioxide alone, asecond strong caustic extraction and a second chlorine dioxidetreatment. These sequences are conventionally represented as C_(D) EHDEDand C_(D) EDED wherein C represents the treatment with chlorine; Hrepresents treatment with hypochlorite; D represents treatment withchlorine dioxide and E represents extraction at pH values greater than10 with aqueous alkali. Thus, for example, C_(D) represents treatmentwith chlorine in the presence of chlorine dioxide.

Extraction of the chlorinated lignin in the first alkaline extractionstage in the above sequence results in substantial darkening of the pulpcompared to the color of pulp at the end of the chlorination stage and asubstantial amount of the chemicals employed in subsequent stages arenecessary simply to remove the color added by the alkaline extraction.

The use of caustic soda as the alkali in the extraction stages ofmultistage chlorine based bleaching has long been traditional because itwas efficient in removing the reaction products of lignin and chlorineor chlorine dioxide and because it was readily available and relativelyinexpensive, being the coproduct of the manufacture of chlorine frombrine. The industrial use of chlorine has been declining in recent yearsand, therefore, the price of caustic soda has risen while the supply hasdeclined to the point where occasional shortages have occurred. Effortsto find alternatives for caustic soda in the bleaching field have, tothe inventors knowledge, been limited to the investigation ofalternative alkalis capable of providing aqueous solutions of high pHvalues. Illustrative of these studies is one by Liebergott et. al.reported in Reports in the 69th Annual Meeting, Technical Section,Canadian Pulp and Paper Association, Montreal, 1982, p. A41. whichexamines a number of alkalis as potential substitutes for caustic sodain the chlorine bleaching process and which reaffirms that caustic sodais the best, that alternative alkalis can be employed and that all workbetter when the pH values are higher.

The present invention provides alternative extraction methods andreagents, principally in the first extraction step, for chlorinatedlignin which minimize darkening of pulp during extraction or permit useof less chemicals to reach brightness levels comparable to thoseattainable with the conventional treatment, or which permit theattainment of higher brightness levels with equivalent chemical usagethan those produced by conventional treatment. The invention alsosurprisingly shows that the particular aqueous extractants contemplatedare effective at pH values lower than those taught as necessary by theprior art.

DISCUSSION OF RELEVANT ART

U.S. Pat. No. 1,645,061 represents an early attempt to develop asuitable bleaching process for kraft pulps. This patent attempts toemploy a two-stage sequence involving a treatment with hypochloritebleaching powder avoiding acid pH followed by treatment with bisulfitesolution under conditions where loss of material is minimal. Thebisulfite is actually employed as an anti-chlor to destroy remainingchlorine chemicals. Use of chlorine or chlorine dioxide bleaching stagesare not mentioned and in that era use of chlorine dioxide particularlywas not even contemplated as it was not commercially available. Thepresent invention employs chlorine at acid pH values and intends tocause delignification, hence, material is lost from the pulp.

U.S. Pat. No. 1,828,338 represents another early attempt to usehypochlorite bleaching powder to bleach kraft pulp. In this process theunbleached pulp is first treated with an alkaline perborate solution andthen hypochlorite in the presence of oxalic acid and NaCl. Once againchlorine or chlorine dioxide treatment stages are not suggested orotherwise contemplated.

U.S. Pat. Nos. 3,695,995 and 3,720,577 illustrate the use of peraceticacid as the second stage in bleaching processes employing oxygen orchlorine dioxide as the oxidant in the first stage. Since the object ofboth patents is to replace the elemental chlorine bleaching process forkraft pulp, use of peracetic acid in connection with a chlorinebleaching process is not suggested or taught.

U.S. Pat. No. 4,119,486 teaches the use of cationic surfactants in anozonated oxygen bleaching process for kraft pulp. That it does not teachways of replacing the alkaline extraction stage in a chlorine bleachingprocess is self-evident.

SUMMARY OF THE INVENTION

The invention provides a process for bleaching kraft pulp comprising:

(a) treating an aqueous slurry of kraft wood pulp with a bleachingeffective amount of chlorine and a cellulose protective amount ofchlorine dioxide;

(b) treating the chlorine and chlorine dioxide treated pulp of step (a)above with an effective amount of a chlorinated lignin solubilizingagent selected from the group consisting of aqueous solutions of loweralkanoic peracids, aqueous solutions of sulfites, and liquid nonaqueoussolvents selected from di-lower alkyl ethers, lower alkanols, loweralkylene polyols, liquid aromatic compounds, liquid lower alkylssubstituted with cyano, nitro, fluoro, chloro, bromo or mixturesthereof, liquid cycloalkanes, liquid cycloalkanes having one or morecarbon atoms replaced by oxygen, sulfur or nitrogen, lower alkanoylamides or di-lower alkyl ketones.

The tangible embodiments produced by the process of the inventionpossess the inherent applied use properties of being pulps suitable forforming sheets of paper or paperboard having improved brightness overcomparable pulps bleached with chlorine in the presence of a celluloseprotective amount of chlorine dioxide and then extracted with aqueousalkali at pH values above 10. Thus, further bleaching to high brightnessvalues may be performed employing a lesser quantity of chemicals thanotherwise would be required. In addition, chlorolignin recovered fromnon-aqueous chlorolignin solvents may be employed as a base for themanufacture of adhesives for gluing cellulose materials such as wood,plywood, particle-board and the like. U.S. Pat. No. 4,292,216 isillustrative of glues based on chlorolignin.

Special mention is made of embodiments of the invention wherein furtherbleaching is continued by sequences comprising:

(a) treatment with hypochlorite followed by treatment with chlorinedioxide, extraction with aqueous alkali and a final treatment withchlorine dioxide;

(b) treatment with chlorine dioxide, extraction with aqueous alkali anda final treatment with chlorine dioxide;

(c) treatment with hypochlorite, followed by treatment with chlorinedioxide, extraction with a chlorinated lignin solubilizing agent asdefined herein above, and a final treatment with a chlorine dioxide; and

(d) treatment with chlorine dioxide, extraction with a chlorinatedlignin solubilizing agent as hereinabove defined and a final treatmentwith chlorine dioxide.

Special mention is also made of preferred embodiments of the inventionwherein the chlorinated lignin solubilizing agent is an aqueous solutionof a lower alkanoic peracid and most preferably wherein the loweralkanoic acid is peracetic acid. Special mention is also made ofpreferred embodiments of the invention wherein the chlorinated ligninsolubilizing agent is an aqueous solution of sulfite ion, mostpreferably an aqueous solution of sulfite ion having a pH value of fromabout 3.0 to about 10.0.

Special mention is also made of particular embodiments of the inventionwherein the chlorinated lignin solubilizing agent is a liquidnon-aqueous solvent as defined hereinabove, preferably one wherein theliquid non-aqueous solvent is a liquid cycloalkane having one or morecarbon atoms replaced by oxygen, sulfur or nitrogen, most preferably onewherein the said cycloalkane comprises tetrahydrofuran or sulfolane.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The manner of practicing the process of the invention to producespecific embodiments thereof will now be illustrated with references toa process for extraction of chlorinated white pine kraft pulp ofstarting Kappa No. 30 to 32 with aqueous solutions of sulfite ion.

White pine pulped by standard sulfate (kraft) pulping techniques to aKappa number of about 30 to about 32 may be treated in standard fashionwith chlorine in the presence of chlorine dioxide, conveniently bytreating an about 3% consistency slurry of white pine kraft pulp ofabout 30 to 32 Kappa number with chlorine at a solution concentration ofabout 6.7% by weight in the presence of chlorine dioxide at a celluloseprotective solution concentration of about 0.1% by weight at slightlyelevated temperature, conveniently at about 30° C., for a short periodof time, conveniently about one hour. After the chlorine-chlorinedioxide treatment the pulp may be separated from the treatment solutionby standard means, such as filtration and washed with water to removeresidual water soluble treatment chemicals and treatment products in theusual way. The washed pulp may then be treated, as an about 10%consistency slurry, with about 3% by weight in solution of sodiumbisulfite at alkaline pH, conveniently about pH 8 at the beginning oftreatment, at moderately elevated temperature, conveniently about 70°C., for a moderate period of time, conveniently for about one and onehalf hours. Following the aforementioned treatment, the pulp may againbe separated from the treatment solution by conventional means, such asfiltration, and washed to remove residual treatment chemicals and watersoluble reaction products.

At this stage, the pulp, which has a brightness higher than pulpsimilarly treated but subjected to a conventional alkaline extractionrather than extraction with sodium bisulfite solution, may be formedinto paper by conventional means or it may be further bleached orbrightened by conventional bleaching techniques, such as one or morechlorine dioxide treatment steps, interspersed with any desiredconventional washings and extractions of chlorine dioxide reactionproducts from the pulp with aqueous alkali or the chloro-ligninsolubilizing agents of this invention. The amount of later stagebleaching chemicals required to reach a specific brightness level willbe less than the amount of chemicals required to reach the samebrightness level starting with a pulp which has been subjected toconventional alkaline extraction after initial chlorine treatment.

In addition to the sodium bisulfite illustrated hereinabove as achlorolignin solubilizing agent, other chlorolignin solubilizing agentscontemplated by the invention include other alkali metal sulfites andbisulfites in aqueous solutions having pH values ranging from about 3 toabout 9. Also suitable are lower alkanoic peracids or water solublesalts of such peracids, preferably peracetic acid and its salts inaqueous solutions having pH values ranging from about 3 to about 7 andvarious various liquid nonaqueous solvents selected from the groupconsisting of di-lower alkylethers, such as di-ethyl ether anddi-isopropyl ether, lower alkanols, such as methanol, ethanol, normaland isopropanol, lower alkylene polyols, such as ethylene glycol,glycerol, hexamethylene glycol, liquid aromatic compounds, such asbenzene, toluene, xylene, pyridine, thiophene, liquid lower alkylssubstituted with cyano, nitro, fluoro, chloro, bromo or mixturesthereof, such as acetonitrile, nitroethane, perfluorokerosene,hexachloroethane, tetrabromo ethane, cycloalkanes, such as cyclohexane,cyclopentane, methyl cyclohexane, cycloalkanes having one or more carbonatoms replaced by oxygen, sulfur or nitrogen, such as tetrahydrofuran,piperidine, sulfolane, tetrahydro thiophene, lower alkanoyl amides, suchas acetamide, di-lower alkyl ketones, such as acetone, methylethylketoneand mixtures thereof. Extraction with peracetic acid may be followedwith a mild alkaline extraction, from about pH 8.0 to about pH 11.0 forfurther enhancement of delignification.

As used herein and in the appended claims, the term bleaching effectiveamount of chlorine means from about 6% by weight to about 9% by weight,preferably from about 7% by weight, to about 8% by weight, the termcellulose protective amount of chlorine dioxide means from about 0.1% byweight to about 0.5% by weight, preferably from about 0.1% by weight toabout 0.2% by weight, the term effective amount of a chlorinated ligninsolubilizing agent means from about 1.9% by weight to about 7.6% byweight of sulfite ion, preferably from about 1.9% by weight to about3.3% by weight of sulfite ion, from about 0.16% by weight to about 4.0%by weight of --CO₃ H radical, preferably from about 0.2% by weight toabout 0.6% by weight of --CO₃ H radical, that is, for example, fromabout 0.2% by weight to about 5.0% by weight of peracetic acid,preferably from about 0.25% by weight to about 0.75% by weight peraceticacid, or for liquid non-aqueous solvents from about 1 liter per 100 goven dry pulp at about a 33% consistency to about 2 liter. The termeffective amount of chlorine dioxide means from about 1.0% by weight toabout 3.0% by weight, preferably from about 1.5% by weight to about 2.0%by weight, the term effective amount of sodium hyprochlorite means fromabout 1.5% by weight to about 2.5% by weight, preferably from about 1.7%by weight to about 2.0% by weight, the term effective amount of anaqueous solution of an alkali metal hydroxide means from about 3% byweight to about 6.0% by weight NaOH, preferably from about 3.5% byweight to about 4.5% by weight NaOH. The above weight % effectiveamounts and cellulose protective amounts are stated on an oven dry pulpbasis. It will be understood by one of skill in the art that in thebleaching sequences described herein and in the appended claims, aconventional water wash between each specifically enumerated separatestage is contemplated unless the context specifically requiresotherwise.

The following examples further illustrate the best mode contemplated bythe inventors for the practice of their invention.

In the following examples unless specifically stated otherwise whitepine (softwood) kraft pulps of 30 to 32 Kappa number were the pulpspecies employed. After the indicated treatment of the chlorinated pulpsthe pulps were then washed with water and further bleached by a DEDsequence.

Unless specifically stated otherwise the following conditions wereemployed for C_(D) EDED bleaching and also for C_(D) E.sub.(modified)DED bleaching with all bulk quantities by weight

C_(D) Stage: Cl₂ =0.225×Kappa No., ClO₂ =0.1%, 3% Pulp Consistency, 30°C., 1 hour

E₁ Stage: 3.5% NaOH, 10% Pulp Consistency, 70° C., 1.5 hr.

D₁ Stage: 1.5% ClO₂, 10% Pulp Consistency, 60° C., 3 hr.

E₂ Stage: 0.6% NaOH, 10% Pulp Consistency, 70° C., 1.5 hr.

D₂ Stage: 0.5 ClO₂, 10% Pulp Consistency, 60° C., 3 hr., SO₂ /H₂ O wash

From the above it will be apparent that in general when employed torepresent the stages in a bleaching sequence, "C_(D) " represents a pulptreatment step wherein the pulp is treated with an aqueous solution ofchlorine in the presence of a minor amount of chlorine dioxide as aviscosity preserver, "E" represents a pulp treatment step wherein thepulp is treated with aqueous alkali to solubilize and remove aqueousalkali soluble materials formed as a result of a prior treatment step;"D" represents a treatment of pulp with aqueous chlorine dioxide, and"E.sub.(modified) " represents a treatment of the pulp with one of thechlorinated lignin solubilizing agents contemplated by this invention.

EXAMPLE 1

All softwood pulps or softwood pulps containing some northern hardwoodpulp as indicated in Table 1, after treatment in the C_(D) stage wereextracted at 70° C. for 1.5 hr employing 3% solutions of sodium sulfite,sodium bisulfite, or SO₂ at the various starting and finishing pH levelsshown in Table 1. The results are tabulated in Table 1. It will be notedthat in certain cases chlorination was performed in the absence ofchlorine dioxide. The results on pulp viscosity are apparent.

Properties of handsheets formed according to TAPPI procedue T-205 om 81from certain of the pulps of Table 1 are shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    BLEACHING SEQUENCE/         BRIGHTNESS (REVERTED),                                                                        VISCOSITY,                        MODIFICATION IN                                                                              PERMANGANATE NO.                                                                           % GE            cP                                EXTRACTION STAGE*                                                                            E.sub.1                                                                           E.sub.2                                                                          FINAL E.sub.1                                                                           E.sub.2                                                                           FINAL   E.sub.2                                                                           FINAL                         __________________________________________________________________________    (A) Starting Pulp:                                                            SWD, 29.8 Kappa                                                               1.  C.sub.D EDED: control                                                                    4.4 0.7                                                                              0.1   39.1                                                                              76.4                                                                              89.5                                                                              (88.0)                                                                            --  26.8                              1s + E, Ph 12/11                                                          2.  C.sub.D E.sub.S DED:                                                                     5.9 1.0                                                                              0.1   47.3                                                                              70.7                                                                              89.6                                                                              (86.8)                                                                            --  26.1                              sulfite, pH 7/5                                                           3.  C.sub.D E.sub.S DED:                                                                     4.8 1.0                                                                              0.1   44.2                                                                              72.3                                                                              88.7                                                                              (86.8)                                                                            --  26.0                              sulfite, pH 8/7                                                           4.  C.sub.D E.sub.S DE.sub.S D:                                                              5.0 1.2                                                                              0.9   46.5                                                                              76.7                                                                              90.0                                                                              (86.3)                                                                            --  18.0                              sulfite, pH 7/5                                                           (B) Starting Pulp:                                                            70% SWD & 30%                                                                 HWD 25.4 Kappa                                                                1.  C.sub.D EDED: control                                                                    3.0 0.7                                                                              0.5   41.8                                                                              76.4                                                                              89.1                                                                              (87.0)                                                                            --  23.8                          2.  C.sub.D E.sub. S DED:                                                                    3.9 1.2                                                                              1.0   48.0                                                                              70.5                                                                              87.9                                                                              (85.0)                                                                            --  17.4                              sulfite, pH 7/4                                                           3.  C.sub.D E.sub.S DED:                                                                     6.9 -- 0.9   40.8                                                                              60.2                                                                              83.5                                                                              (78.7)                                                                            --  18.5                              sulfite, no                                                                   wash between                                                                  C & E, pH 6/3                                                             4.  C(SO.sub.2)DED: no                                                                       9.2 -- 0.7   37.5                                                                              54.5                                                                              80.3                                                                              (77.3)                                                                            --  15.0                              wash after                                                                    C stage                                                                   5.  CE.sub.S DED:                                                                            4.2 1.2                                                                              0.9   47.8                                                                              69.9                                                                              88.5                                                                              (85.5)                                                                            --  12.1                              bisulfite,                                                                    pH 5/2                                                                    (C) Starting Pulp:                                                            SWD, 32.8 Kappa                                                               1.  CEDED: control                                                                           3.4 1.1                                                                              0.5   37.2                                                                              76.5                                                                              88.2                                                                              (86.6)                                                                            24.4                                                                              --                            2.  CE.sub.S DED:                                                                            3.7 1.2                                                                              1.0   42.2                                                                              71.6                                                                              87.4                                                                              (84.6)                                                                            24.7                                                                              --                                sulfite, pH 9/7                                                           3.  CE.sub.S DE.sub.S D:                                                                     3.6 1.2                                                                              0.8   41.5                                                                              70.5                                                                              86.4                                                                              (83.4)                                                                            26.4                                                                              --                                sulfite, pH 9/7                                                           4.  C.sub.D EDED: control                                                                    3.2 1.0                                                                              0.6   39.0                                                                              78.1                                                                              88.6                                                                              (86.6)                                                                            30.6                                                                              --                            5.  C.sub.D E.sub.S DE.sub.S D:                                                              3.9 1.0                                                                              0.9   42.3                                                                              69.9                                                                              87.5                                                                              (84.1)                                                                            31.0                                                                              --                                sulfite, pH 9/7                                                           __________________________________________________________________________     Notes for Table 1                                                             *E  3.5% NaOH.                                                                E.sub.S  3% Na.sub.2 SO.sub.3 or NaHSO.sub.3, approximately 0.8% NaOH         added when pH 8-9.                                                       

                  TABLE 2                                                         ______________________________________                                                         VIS-    HANDSHEET                                                     BRIGHT- COS-     PROPERTIES                                          BLEACHING  NESS      ITY     BL,            CSF,                              SEQUENCE   % GE      cP      km   BF   TF   ml                                ______________________________________                                        (A*) 1. C.sub.D EDED                                                                     89.5      26.8    12.7 103.8                                                                              87.9 475                               2. C.sub.D E.sub.S DED                                                                   89.6      26.1    13.7 100.2                                                                              89.7 450                               4. C.sub.D E.sub.S DE.sub.S D                                                            90.0      18.0    12.9  93.4                                                                              82.5 420                               (B*) 1. CEDED                                                                            89.1      23.8    11.6  83.4                                                                              91.7 375                               2. CE.sub.S DED                                                                          87.9      17.4    11.4  81.7                                                                              86.0 390                               ______________________________________                                         Notes for Table 2                                                             *Starting Pulp A and B as in Table 1                                          BL = Breaking length (TAPPI procedure T404)                                   BF = Burst Factor (TAPPI procedure T403)                                      TF = Tear Factor (TAPPI procedure T414)                                       CSF = Canadian Standard Freeness (Technical Section, Canadian Pulp and        Paper Assoc. procedure Cl)                                               

EXAMPLE 2

The effects of varying the concentration of sulfite, pH, timetemperature and pulp consistency compared to standard controls is shownin Table 3.

                                      TABLE 3                                     __________________________________________________________________________             C.sub.D E.sub.S DED     C.sub.D EDED                                          1   2   3*  4*  5   6   7   8                                        __________________________________________________________________________    First Extraction                                                              Na.sub.2 SO.sub.3                                                                       3   6  12  12  12  12  --  --                                       Applied, (%)                                                                  NaOH     0.2 --  --  --  --  --  3.5 3.5                                      Applied, (%)                                                                  Pulp Consis-                                                                           10  10   4   4   4   4  10  10                                       tency, (%)                                                                    Temp, (°C.)                                                                     70  90  110 125 150 175 70  70                                       Time, min                                                                              90  90  60  60  60  60  90  90                                       Initial/ 7.1/4.6                                                                           7.5/7.0                                                                           7.8/6.2                                                                           7.8/6.2                                                                           7.8/6.0                                                                           7.8/5.6                                                                           12/11                                                                             12/11                                    Final pH                                                                      Permanganate                                                                           5.9 5.7 3.2  2.6                                                                              2.1 1.4 3.0 3.0                                      No.                                                                           Brightness,                                                                            47  44  45  48  49  48  38  38                                       (% GE)                                                                        First Dioxide                                                                 ClO.sub.2                                                                              1.5     1.0     1.0 0.5 0.5 1.0                                      Applied, (%)                                                                  ClO.sub.2                                                                              93      77      68  70  100 86                                       Consumed, (%)                                                                 Second Extraction                                                             Permanganate                                                                           1.0             0.7 0.4 0.8 0.5                                      No.                                                                           Brightness,                                                                            71      77.6    78  81  67  77                                       (% GE)                                                                        Second Dioxide                                                                ClO.sub.2                                                                              0.5     0.5     0.5 0.5 0.5 0.5                                      Applied, (%)                                                                  ClO.sub.2                                                                              65          53  45  40  86  80                                       Consumed, (%)                                                                 Brightness,                                                                            89.6        89.6                                                                              88.4                                                                              89.3                                                                              86.5                                                                              88.6                                     (% GE)                                                                        Reverted 86.8        86.9                                                                              86.0                                                                              86.3                                                                              83.2                                                                              85.9                                     Brightness,                                                                   (% GE)                                                                        Viscosity,                                                                             26.1        20.5                                                                              18.6                                                                              14.8                                                                              28.7                                                                              24.4                                     (cP)                                                                          __________________________________________________________________________     Notes for Table 3                                                             *Pulps in examples 3 and 4 were combined after the 1st extraction stage       and then bleached by DED stages.                                         

EXAMPLE 3

Softwood pulps are bleached employing an aqueous solution of peraceticacid (Pa) in place of the aqueous alkali (E) employed in a normal C_(D)EDED bleaching sequence. That is, a C_(D) PaDED sequence is employed.Standard C_(D) EDED bleaching sequences are run on similar pulps. Theconditions employed and the results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                                         E.sub.1 /Pa STAGE                                                             PROPERTIES                               TRIAL                                    BRIGHTNESS,                          No.                                  P No.                                                                             % GE                                 __________________________________________________________________________    1a   Control E.sub.1 (3.5% NaOH)     3.0 37.6                                 1b   Control E.sub.1 (3.5% NaOH)     3.0 37.6                                 PERACETIC ACID TREATMENT OF C.sub.D PULP                                                       pH                                                                % Pa    %   INITIAL/                                                                            TEMP,                                                                              WASHING/                                               (CONSUMED)                                                                            NaOH                                                                              FINAL °C.                                                                         EXTRACTION                                        2a    0.5 (0.25)                                                                           None                                                                              3.2/2.8                                                                             50   Hot Water                                                                              5.4 45.7                                                             (75-80° C.)                                                            Wash (HWW)                                                                    Further                                                                       wash with                                                                     Na.sub.2 CO.sub.3 sol.                                                        (0.1%)                                            2b    0.5 (0.40)                                                                           0.5 4.0/3.0                                                                             50   HWW      5.5 46.6                                 3a    0.5 (0.23)                                                                           None                                                                              3.2/2.5                                                                             77   HWW      5.7 44.0                                 3b    0.5 (0.44)                                                                           0.5 4.2/2.8                                                                             77   HWW      5.0 45.0                                 3c    0.5 (0.43)                                                                           1.0 4.3/3.3                                                                             77   HWW      4.0 48.2                                 3d   0.5 (0.5)                                                                             2.0 6.5/4.5                                                                             77   HWW      2.8 51.5                                 3e   0.5 (0.5)                                                                             2.0 6.5/4.5                                                                             77   HWW      2.8 51.5                                 3f   0.5 (0.5)                                                                             2.0 6.5/4.5                                                                             77   Alk. Extr.                                                                             2.0 --                                                               (1% NaOH)                                         3g   0.5 (0.5)                                                                             2.0 6.5/4.5                                                                             77   Alk. Extr                                                                              2.0 --                                                               (1% NaOH)                                         4a   0.25 (0.25)                                                                           2.0 6.6/5.0                                                                             77   HWW      4.0 40.5                                 4b   0.25 (0.25)                                                                           2.0 6.6/5.0                                                                             77   HWW      4.0 40.5                                 __________________________________________________________________________                                  FINAL (D.sub.2)                                 D.sub.1 E.sub.2 STAGE         PROPERTIES                                                PROPERTIES               Brightness                                 TRIAL         BRIGHTNESS,                                                                            VISCOSITY,  % GE    VISCOSITY,                         NO.  % ClO.sub.2                                                                        P No.                                                                             % GE     cP     P No.*                                                                             (REVERTED)                                                                            cP                                 __________________________________________________________________________    1a   1.0  0.5 76.9     --     0.3  88.6 (85.9)                                                                           24.4                               1b   0.5  0.8 66.7     --     0.3  86.5 (83.2)                                                                           28.7                               2a   1.0  1.4 61.2     --     0.5  87.1 (84.2)                                                                           19.8                               2b   1.0  1.6 60.5     --     --   86.2 (83.2)                                                                           20.4                               3a   1.0  1.2 62.5     13.1   --   --      --                                 3b   1.0  1.1 65.5     17.9   --   --      --                                 3c   1.0  1.0 68.3     23.4   --   87.6 (85.2)                                                                           20.1                               3d   1.0  --  72.1     --     0.3  89.0 (85.6)                                                                           22.5                               3e   0.5  --  67.2     --     0.5  87.7 (83.6)                                                                           22.3                               3f   0.5  0.8 75.9     27.9   --   88.2 (86.3)                                                                           23.8                               3g    0.25                                                                              1.0 74.0     28.8   --   87.6 (85.5)                                                                           24.2                               4a   1.0  1.2 67.5     28.1   0.6  88.3 (85.7)                                                                           23.7                               4b   0.5  1.8 57.7     --     0.6  85.4 (82.5)                                                                           29.2                               __________________________________________________________________________     Note for Table 4                                                              *Permanganate Number                                                     

EXAMPLE 4

Pulps are bleached as shown in Table 5. The conditions employed and theresults obtained are also tabulated in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                             PULP PROPERTIES                                                                                   FINAL STAGE                                 CHLORI-       T/E.sub.1 STAGE                                                                         2nd E STAGE                                                                             BRIGHT-                                     NATION                                                                              TOTAL   BRIGHT-   BRIGHT-   NESS, VIS-                              TEST                                                                              TEMP, CIO.sub.2                                                                             NESS,     NESS,     % GE  COSITY                     SEQUENCE                                                                             NO. °C.                                                                          APPLIED, %                                                                            % GE  P No.                                                                             % GE  P No.                                                                             (REV) (cP)                       __________________________________________________________________________    C.sub.D TDED*                                                                        1   30    1.5           5.9 49.6  2.7 81                               C.sub.D TDED                                                                         2   30    2.0               57.5  1.8 85.4  22.6                                                                    (82.1)                           C.sub.D TDED                                                                         3   55    2.0     42        62.5  1.3 88.6  21.7                                                                    (86.1)                           Conven-                                                                              4   30    1.5     38        76.0  0.8 88.6  24.0                       tional                                       (86.0)                           C.sub.D EDED                                                                  Conven-                                                                              5   55    2.0     42        77.4  0.5 90.2  25.0                       tional                                                                        C.sub.D EDED                                                                  __________________________________________________________________________     Note for Table 5                                                              *T = Tetrahydrofuran (THF) extraction at 30°  C., 21 THF/100 g pul     (oven dry basis) at 33% consistency.                                     

EXAMPLE 5

A comparison is made of strength properties of handsheets made frompulps bleached as illustrated in the prior examples.

The bleaching sequences and the results are tabulated in Table 6.

                  TABLE 6                                                         ______________________________________                                                                      HANDSHEET                                                  BRIGHT-   VISCOS-  PROPERTIES                                      BLEACHING* NESS,     ITY,     (400 CSF)                                       SEQUENCE   % GE      cP       BL, km BF   TF                                  ______________________________________                                        C.sub.D EDED                                                                             88.6      24.4     11.9   93.3 81.0                                C.sub.D PaDED                                                                            87.1      19.8     10.8   89.4 72.0                                C.sub.D PaDED                                                                            88.3      23.7     12.2   91.7 84.7                                C.sub.D E.sub.S DED** (a)                                                                89.6      20.5     11.2   82.0 80.0                                C.sub.D E.sub.S DE.sub.S D** (b)                                                         90.0      18.0     12.9   93.4 82.5                                C.sub.D TDED                                                                             88.6      21.7     10.7   91.0 77.0                                ______________________________________                                         Notes for Table 6                                                             *E   with NaOH                                                                E.sub.S  Extraction with Na.sub.2                                             Pa  Extraction with peracetic acid                                            T  Extraction with tetrahydrofuran                                            **Sulfite treatment at 110-125° C. in (a) and at 70° in (b)

EXAMPLE 6

Northern softwood kraft pulp of kappa No. 29 is bleached by thesequences and under the conditions shown in Table 7. The results foundare tabulated therein.

                  TABLE 7                                                         ______________________________________                                                                        VIS-                                                       BRIGHTNESS,        COSITY                                                     % GE (reverted)                                                                          P No.   cP                                            STAGE          E.sub.1  E.sub.2                                                                       Final   E.sub.1                                                                             Final                                   ______________________________________                                         (1) C.sub.D EDED (Control)                                                                  36.5 77.4                                                                              87.0     3.7   24.5                                   D.sub.1 = 1% ClO.sub.2, (85.2)                                                D.sub.2 = 0.5% ClO.sub.2                                                      E.sub.1 3.5% NaOH                                                             (2) C.sub.D PaEDED                                                                           48.1 74.8                                                                              87.0    2.5   22.7                                    Pa; 0.5% Pa, 2% NaOH,   (85.2)                                                77° C., 90 min.                                                        pH = 6.4/4.6                                                                  E.sub.1 ; 1.5% NaOH, 70° C.,                                           30 min                                                                        D.sub.1 = 0.25% ClO.sub.2,                                                    D.sub.2 = 0.50% ClO.sub.2                                                     (3) C.sub.D PaEDED                                                                           48.4 79.8                                                                              87.5    2.7   22.3                                    PaE; as in #2 above             (85.0)                                        D.sub.1 = 0.5% ClO.sub.2,                                                     D.sub.2 = 0.25%                                                               (4) C.sub.D PaEDED                                                                           48.5 78.3                                                                              87.5    2.6   22.3                                    Pa; as above #2 & 3     (86.0)                                                E.sub.1 ; 3% NaOH, 70° C.                                              60 min                                                                        D.sub.1 = 0.5% ClO.sub.2,                                                     D.sub.2 = 0.25% ClO.sub.2                                                     ______________________________________                                    

EXAMPLE 7

Northern softwood kraft pulp was bleached employing the sequences shownin table 8 under the conditions specified therein. The results tabulatedin table 8 illustrate that extraction of chlorine bleached Kraft woodpulps with aqueous peracetic acid at pH7 or below with aqueous sulfiteat pH7 or below, or with solvents such as tetrahydrofuran permits theuse of lesser quantities of sodium hydroxide and/or chlorine dioxide toattain brightness and physical properties comparable to those of pulpstreated by alkaline extraction at pH values greater than 10.

While the mechanism of action is not completely known and withoutlimitation thereto it appears that peracetic acid reacts withchlorolignin in chlorinated Kraft pulp thereby activating the residualchlorolignin permitting easier removal from the pulp. Thus, lesschlorine dioxide is required for further bleaching.

                                      TABLE 8                                     __________________________________________________________________________                                                   HANDSHEET PROPERTIES                       CHEMICALS              FINAL STAGE (400 CSF)                                  APPLIED                        VIS-                                                                              BREAK-                                     AFTER C.sub.D STAGE,                                                                     1st EXTR. STAGE                                                                           BRIGHTNESS                                                                            COS-                                                                              ING         TEAR               TEST                                                                              BLEACHING                                                                             % ON PULP  BRIGHTNESS, % GE    ITY LENGTH                                                                              BURST FAC-                NO.                                                                              SEQUENCE                                                                              NaOH ClO.sub.2                                                                           % GE     P #                                                                              (REVERTED)                                                                            cP  (km)  FACTOR                                                                              TOR                __________________________________________________________________________    1.  C.sub.D PaDED                                                                         2.6  1.5   51.5     2.8                                                                              89.0 (85.6)                                                                           22.6                                                                              --    --    --                 2.  C.sub.D (PaE)DED                                                                      3.6  0.75-1.0                                                                            48.0     2.0                                                                              88.2 (86.2)                                                                           23.8                                                                              12.2  91.7  84.7               3.  C.sub.D E.sub.S DED                                                                   0.6  1.5   49.0     2.1                                                                              89.6 (86.9)                                                                           20.5                                                                              11.2  82.0  80.0               4.  C.sub.D TDED                                                                          0.6  2.0   42.0     -- 88.6 (86.1)                                                                           21.7                                                                              10.7  91.0  77.0               5.sup.a                                                                           C.sub.D PaTDD                                                                         2.0  1.5   --       -- 85.5    23.1                                                                              --    --    --                 6.  C.sub.D EDED                                                                          4.1  1.5   37.6     3.0                                                                              88.6 (85.9)                                                                           24.4                                                                              11.9  93.3  81.0               __________________________________________________________________________     Notes for Table 8                                                             Pa  Extraction with 0.5% peracetic acid, pH (initial/final) 6.5/4.5,          77° C., 2 hr, followed by hot water wash in Test No. 1 and by a        mild alkaline extraction in Test No. 2. Wash between Pa and E stages is       optional.                                                                     E.sub.S  Extraction with sodium sulfite (3-12%), 70-150° C., pH        7/5.                                                                          T  Extraction with tetrahydrofuran (21/100 g pulp). Chlorination temp. wa     55° C. in Test Nos. 4 and 5 (30° C. in all other Tests). 9%     Cl.sub.2 on pulp. in Test No. 5, and 7% in other Tests.                       .sup.a This test represents a fully nonalkaline sequence. A repeat of thi     bleaching experiment starting with 36 kappa pulp gave bleached pulp of        brightness 85.8% GE, reverted brightness 81.6% GE, and viscosity 20.4.   

EXAMPLE 8

Softwood kraft pulp, kappa No. 36, is bleached according to thebleaching sequences and under the conditions shown in Table 9. Theresults are tabulated therein. In the bleaching sequences shown, Hrepresents a hypochlorite bleaching step.

                                      TABLE 9                                     __________________________________________________________________________                      C.sub.D EHD                                                                   (control)                                                                              C.sub.D PaHD                                                                       C.sub.D (PaE)*                                __________________________________________________________________________    1st Stage                                                                     % Cl.sub.2 /% ClO.sub.2                                                                         0.225 × kappa/0.1                                     Time, min/Temp. (°C.)                                                                    60/30                                                       2nd Stage                                                                     Sodium Hydroxide, (%)                                                                           3.5      2.0   3.5**                                        Peracetic Acid    0.0      0.5  0.5                                           Time, min/Temp. (°C.)                                                                    90/77    90/77                                                                              90/77 30/70***                                pH Initial/Final  12/11    7/5  7/5 12/11****                                 Properties                                                                    P. No.            4.2      3.0  3.0                                           Brightness (% GE) 34.8     37.4 43.1                                          3rd Stage                                                                     Sodium Hypochlorite                                                                             1.7      1.7  1.7                                           Time, min/Temp. (°C.)                                                                    120/50   120/50                                                                             120/50                                        pH Initial/Final  11.2/9.5 11/8.2                                                                             11.3/9.4                                      4th Stage                                                                     % ClO.sub.2       0.5      0.5  0.5                                           Time, min/Temp. (°C.)                                                                    180/60   180/60                                                                             180/60                                        Properties                                                                    Brightness % GE (Reverted % GE)                                                                 84.5 (82.3)                                                                            84.3 (81.9)                                                                        87.7 (85.2)                                   Viscosity (cP)    20.1     17.2 17.2                                          __________________________________________________________________________     Notes for Table 9                                                             *No wash between Pa and E stages.                                             **2.0% NaOH in Pa stage, 15% NaOH in E stage.                                 ***Initial two numbers represent time/temperature in Pa stage, second two     numbers represent time/temperature in E stage.                                ****Initial two numbers represent pH values for Pa stage, second two          numbers represent pH values for E stage.                                 

EXAMPLE 9

Northern softwood kraft pulp (Kappa No. 30) was bleached by thesequences and under the conditions shown in Table 10. The results aretabulated therein. In the bleach sequences A represents the addition ofmineral acid to adjust pH.

                                      TABLE 10                                    __________________________________________________________________________                 Control                                                                       C.sub.D EDED                                                                  (Conv.)                                                                             C.sub.D EC.sub.D ED                                                                 C.sub.D AC.sub.D ED                                                                 C.sub.D PaC.sub.D ED                           Sequence     1     2     3     4   5   6                                      __________________________________________________________________________    Test No                                                                       1st Stage                                                                     % Cl.sub.2 /% ClO.sub.2                                                                    6.8/0.1                                                          Temp. (°C.) Time, (min.)                                                            30/60                                                            2nd Stage                                                                     (90 min, 70° C.)                                                       Sodium Hydroxide, (%)                                                                      3.5   3.5   --    2.0 2.0 2.0                                    Peracetic Acid, (%)                                                                        --    --    --    0.5 0.5 0.5                                    Time, (min)/Temp (°C.)                                                              90/70 90/70 90/70 90/70                                                                             90/70                                                                             90/77                                  pH On/Off    11.6/10.8                                                                           11.6/10.8                                                                           5.5/5.5                                                                             7.5/5.3                                                                           7.5/5.3                                                                           7.2/4.8                                P. No.       2.9   2.8   --    2.6 2.3 --                                     Brightness (% G.E.)                                                                        40    40    --    44  47  --                                     3rd Stage                                                                     % Cl.sub.2   --    2.5   2.5   2.5 2.0 2.5                                    % ClO.sub.2  1.0   0.25  0.25  0.25                                                                              0.2 0.1                                    Temp (°C.) Time, (min)                                                              60/180                                                                              50/60 50/60 50/60                                                                             35/60                                                                             50/60                                  4th Stage                                                                     Sodium Hydroxide, %                                                                        0.6                                                              Temp (°C.) Time, (min)                                                              70/90                                                            % G.E.       78    67    57    64  64  65                                     5th Stage                                                                     % ClO.sub.2  0.5                                                              Temp (°C.) Time, (min)                                                              60/180                                                           Brightness (% G.E.)                                                                        88.4  87.5  86.8  88.3                                                                              87.6                                                                              88.1                                   Reverted     87.7  87.0  83.0  87.4                                                                              86.0                                                                              85.4                                   Brightness (% G.E.)                                                           Viscosity, (cP)                                                                            24.3  20.9  --    18.9                                                                              22.1                                                                              17.5                                   __________________________________________________________________________

EXAMPLE 10

Northern softwood kraft pulp after treatment with chlorine in thepresence of chlorine dioxide is further treated as set forth in Table 11under the conditions set forth therein to obtain the results shown.

                                      TABLE 11                                    __________________________________________________________________________               Control                                                            Stage/Test No.                                                                           1    2    3     4    5                                             __________________________________________________________________________    Pa % Pa applied/                                                                         --   0.5/0                                                                              0.5/0 1.25/19                                                                            1.25/19                                       % Pa Resid.                                                                   pH initial/pH final                                                                      --   7.2/5.2                                                                            7.2/5.2                                                                             6.8/4.5                                                                            6.8/4.5                                       Time (min) Temp. (°C.)                                                            --   90/77                                                                              90/77 90/77                                                                              90/77                                         Temp. (°C.)                                                            Brightness (% GE)                                                                        --   45.4 --    56.6 --                                            P. No. viscosity                                                                         --    5.4/25.6                                                                          --     2.5/25.0                                                                          --                                            E % NaOH   3.5  --   1.5   --   1.5                                           Time, (min/Temp.) °C.                                                             90/70                                                                              --   30/70 --   30/70                                         pH initial/pH final                                                                      11.5/11.0                                                                          --   11.7/11.2                                                                           --   11.2/10.6                                     Brightness (% GE)                                                                        35   --   43.8  --   52.0                                          P. No./Viscosity                                                                          5.4/28.0                                                                          --    3.9/25.0                                                                           --    2.3/24.2                                     D % ClO.sub.2 /% ClO.sub.2                                                               --   1.0/2.6                                                                            1.0/3.1                                                                             1.0/3.5                                                                            1.0/3.9                                       Resid                                                                         Time, min/Temp. °C.                                                               --   180/60                                                                             180/60                                                                              180/60                                                                             180/60                                        pH on/pH off                                                                             --    3.1 3.2    3.3 3.2                                           Brightness (% GE)/                                                                       --   62.4/56.0                                                                          78.2/71.4                                                                           76.6/70.4                                                                          83.3/78.8                                     Reverted Brightness                                                           (% GE)                                                                        P. No./Viscosity                                                                         --    1.5/24.2                                                                           1.1/23.8                                                                            1.4/22.3                                                                           0.9/23.4                                     __________________________________________________________________________           Control                                                                Stage/Test No.                                                                       6    7    8    9    10   11                                            __________________________________________________________________________    Pa % Pa applied                                                                      2.0/20                                                                             2.0/20                                                                             3.0/25                                                                             4.0/26                                                                             5.0/32                                                                             10.0/45                                       % Resid.                                                                      pH initial/                                                                          6.7/4.0                                                                            6.7/4.0                                                                            7.3/4.8                                                                            7.0/4.8                                                                            7.5/5.0                                                                            7.1/4.5                                       pH final                                                                      Time, (min)/                                                                         90/77                                                                              90/77                                                                              90/77                                                                              60/77                                                                              45/77                                                                              30/77                                         Temp °C.                                                               Brightness                                                                           70.5 --   82.4/69.0                                                                          82.8/66.5                                                                          83.4/69.5                                                                          83.7/67.5                                     (% GE)/ Rev.                                                                  Bright.                                                                       (% GE)                                                                        P. No./                                                                               2.2/24.4                                                                          --    1.1/23.0                                                                           0.8/20.8                                                                           0.7/19.1                                                                           0.7/16.7                                     Viscosity                                                                     E % NaOH                                                                             --    1.5  1.5  1.5  1.5  1.5                                          Time, (min)                                                                          --   30/70                                                                              30/70                                                                              30/70                                                                              30/70                                                                              30/70                                         Temp (°C.)                                                             pH initial/pH                                                                        --   10.9/10.4                                                                          11.6/10.4                                                                          11.7/10.4                                                                          11.6/10.3                                                                          11.8/10.4                                     final                                                                         Brightness                                                                           --   62.3 74.2 73.3 75.2 75.1                                          (% GE)                                                                        P. No./                                                                              --    2.0/23.8                                                                           0.4/17.3                                                                           0.4/16.8                                                                           0.3/15.3                                                                           0.2/12.9                                     Viscosity                                                                     D % ClO.sub.2 /% ClO.sub.2                                                           1.0/3.9                                                                            1.0/4.0                                                                            --   --   --   --                                            Resid                                                                         Time, (min)/                                                                         180/60                                                                             180/60                                                                             --   --   --   --                                            Temp (°C.)                                                             pH final                                                                              3.2 3.3  --   --   --   --                                            Brightness                                                                           80.8/76.3                                                                          84.6/79.2                                                                          --   --   --   --                                            (% GE)/                                                                       Rev. Bright.                                                                  (% GE)                                                                        P. No./                                                                               0.9/23.1                                                                           0.6/22.7                                                                          --   --   --   --                                            Viscosity                                                                     __________________________________________________________________________     Notes for Table 11                                                            *The Permanganate number 5.4 for the control C.sub.D E pulp indicates tha     the starting pulp was probably underchlorinated.                              For pH adjustment in Pa stage, 2% NaOH applied in Nos. 2 & 3, and             increasing amounts in Nos. 4-11.                                         

EXAMPLE 12

Softwood pulp is bleached similarly to Example 4 but substitutingsulfolane at 70° to 90° C. for 30 to 60 minutes for tetrahydrofuran toprovide the sequence:

C_(D) [sulfolane] DED.

The bleached pulp had a GE brightness of 86.8%, viscosity 18.8 cp.

The subject matter which applicants regard as their invention isparticularly pointed out and distinctly claimed as follows:
 1. A processfor bleaching kraft pulp comprising:(a) treating an aqueous slurry ofkraft pulp with a bleaching effective amount of chlorine and a celluloseprotective amount of chlorine dioxide; (b) extracting the pulp of step(a) above at an acidic pH and at a temperature range of from about 30°C. to about 77° C. with an effective amount of a chlorinated ligninsolubilizing agent comprising an aqueous solution of a sulfite ion; (c)treating the pulp from step (b) above with an effective amount ofchlorine dioxide or sodium hypochlorite; (d) extracting the pulp fromstep (c) above with an effective amount of an aqueous solution of analkali metal hydroxide or a chlorinated lignin solubilizing agent asdefined in step (b) above; and (e) treating the pulp from step (d) abovewith an effective amount of chlorine dioxide.
 2. A process as defined inclaim 1 wherein the treatment in step (d) is with an aqueous solution ofan alkali metal hydroxide.
 3. A process as defined in claim 2 whereinthe alkali metal hydroxide is sodium hydroxide.
 4. A process as definedin claim 1 wherein the treatment in step (d) is with a chlorinatedlignin solubilizing agent.
 5. A process as defined in claim 1 whereinthe aqueous solution of sulfite ion has a pH of from about 3 to about 5.6. A process as defined in claim 5 wherein the aqueous solution ofsulfite ion contains counter ions derived from sodium.